Weather Glossary - V

V Notch

A radar reflectivity signature seen as a V-shaped notch in the downwind part of a thunderstorm echo. The V-notch often is seen on supercells, and is thought to be a sign of diverging flow around the main storm updraft (and hence a very strong updraft). This term should not be confused with inflow notch or with enhanced V, although the latter is believed to form by a similar process.

Vapour

Gaseous form of a substance.

Variability index

An index for assessing the variability of annual rainfall.

Variability Index = {(90th percentile -10th percentile) / 50th percentile}

Vault

Same as BWER.

Veering winds

Winds which shift in a clockwise direction with time at a given location (e.g., from southerly to westerly), or which change direction in a clockwise sense with height (e.g., southeasterly at the surface turning to southwesterly aloft). The latter example is a form of directional shear which is important for tornado formation. Compare with backing winds.

Vertical velocity

A measure of the upward motion of air in the atmosphere. Vertical velocities are in general around 1 cm.s-1 (compared to 10 m.s-1 for horizontal velocities), but can reach several m.s-1 in thunderstorm updrafts. Often the vertical velocity is displayed in hectopascals per hour. Since pressure decreases with height, negative values of the vertical velocity indicate rising motion in the atmosphere, and positive values indicate sinking air). Large values of the vertical velocity on these charts can (when combined with high moisture level) indicate the potential for heavy rainfall.

Vertically stacked system

A low-pressure system, usually a closed low or cut-off low, which is not tilted with height, i.e., located similarly at all levels of the atmosphere. Such systems typically are weakening and are slow-moving, and are less likely to produce severe weather than tilted systems. However, cold pools aloft associated with vertically-stacked systems may enhance instability enough to produce severe weather.

VIL

Vertically-Integrated Liquid water. A property computed by RADAP II and WSR-88D units that takes into account the three-dimensional reflectivity of an echo. The maximum VIL of a storm is useful in determining its potential severity, especially in terms of maximum hail size.

VIP

Video Integrator and Processor, which contours radar reflectivity (in dBZ) into six VIP levels:

• VIP 1 (Level 1, 18-30 dBZ) - Light precipitation
• VIP 2 (Level 2, 30-38 dBZ) - Light to moderate rain.
• VIP 3 (Level 3, 38-44 dBZ) - Moderate to hevay rain.
• VIP 4 (Level 4, 44-50 dBZ) - Heavy rain
• VIP 5 (Level 5, 50-57 dBZ) - Very heavy rain; hail possible.
• VIP 6 (Level 6, >57 dBZ) - Very heavy rain and hail; large hail possible.

Virga

Streaks or wisps of precipitation falling from a cloud but evaporating before reaching the ground. In certain cases, shafts of virga may precede a microburst; see dry microburst.

Virga from altocumulus.

Visible satellite image

Visible satellite images can only be viewed during the day, since clouds reflecting sunlight are used to create the images. On visible images, clouds show up as white or light grey, the land is normally grey and the oceans dark (some features of the land can be identified). Visible images can reveal low cloud features that infrared images do not show well, such as fog and small cumulus. Visible images are useful to identify developing thunderstorms, before they are detected on radar.

Volume scan

A radar scanning strategy in which sweeps are made at successive antenna elevations (i.e., a tilt sequence), and then combined to obtain the three-dimensional structure of the echoes. Volume scans are necessary to determine thunderstorm type, and to detect features such as WERs, BWERs, and overhang.

Vort Max

[Slang] Short for vorticity maximum. A centre, or maximum, in the vorticity field of a fluid.

Vorticity

A measure of the local rotation in a fluid flow. In weather analysis and forecasting, it usually refers to the vertical component of rotation (i.e. rotation about a vertical axis) and is used most often in reference to synoptic scale or mesoscale weather systems. By convention in the Southern Hemisphere, negative values indicate cyclonic (clockwise) rotation.